The present invention is directed generally to a brake unit having a nonmetallic brake disc of ceramic and a hub.
A brake unit and a manufacturing method are known, for example, from German Patent No. 44 38 455 C1 and from German Patent Application No. 44 38 456 A1. German Patent Application No. 44 38 456 A1 also describes a frictional unit, a frictional body, and a core body, the core body being able to have an extension prolongation with connection orifices in the manner of a nonmetallic hub. The frictional body is made of a Cxe2x80x94C/SiC material, while the core body is constituted of a carbon fiber-reinforced carbon. The core body and the frictional body are joined by an interconnection layer containing essentially silicon carbide.
Brake units are also known,-whose brake discs are constituted of carbon over their entire cross-section. Special measures are required to adapt a toroidally-shaped disc of this kind to the wheel hub. Provision has been made in known methods for a hat- or pot-shaped metal hub. This hub is mechanically coupled by its free rim to the brake disc. This mechanical coupling is achieved using a wreath-type rim of mounting bolts and corresponding entrainment bores or radial slots in the area of the inner rim or the inside of the brake disc, which produce a mechanically loadable form-locking between the brake disc and metal hub. Provision is usually made for six or eight mounting bolts.
What is problematic here is the difference between the expansion coefficient specific to the brake disc (about 3-5xc3x9710xe2x88x926/K) and that specific to the metal hub (about 15-20xc3x9710xe2x88x926/K). For that reason, one has to prevent the parts from sticking, as can occur due to the tension produced by this type of attachment. Rather, radial expansion must still be allowed between the two parts, in spite of the mechanical coupling. Therefore, a suitable play is intentionally permitted between the entrainment bores and the bolt in the radial direction, whereas a precise as possible fit with the bolt diameter is aspired to in the circumferential direction.
The predefined, hub-side position of the bolts, on the one hand, and the disc-side position of the entrainment bores, on the other hand, must, therefore, conform as precisely as possible. However, in spite of all efforts to achieve manufacturing precision, mathematical agreement can not be achieved with any kind of realistic cost outlay. Rather, in practice, it may not be possible to entirely avoid positional tolerances with respect to the bolts, on one hand, and the entrainment bores, on the other hand. As a result, merely two, at the most three, of all the provided bolts are load-bearing in response to mechanical stressing of the brake, whereas a play exists between the remaining bolts and the entrainment bores, preventing these bolts from bearing load.
After a plastic or wear-related deformation has taken place in the area of the bearing bolts, it is possible for the play existing in the area of the other bolts to be reduced, enabling these bolts to be load-bearing as well. When working with carbon discs, sufficient plasticity is given in the area of the entrainment bores, depending on the material used, so that in this case, at the latest following the first extreme mechanical stressing of the brakes, the initially sole three bearing bolts have pressed into the inside face of the corresponding entrainment bores, plastically deforming these bores, eliminating the play between the remaining bolts and their entrainment bores, thus rendering them load-bearing as well.
It is a different situation, however, with rigid ceramic discs. In the just described mechanical coupling between the brake disc and the metal hub, play may be eliminated for the non-bearing bolts, due to wear, or because of a plastic deformation of the bearing bolts; but this can take a very long time.
A solution to this problem, as described in the German Patent Application No. 44 38 456 A1, is to revert to a nonmetallic hub, which would lessen the differences with respect to the expansion coefficients. What is problematic, however, is securing the hub to the brake disc, due to the high mechanical and thermal stressing experienced during braking maneuvers.
An object of the present invention is to improve a brake unit having a nonmetallic disc of ceramic and a metal hub so that in spite of an inflexible ceramic disc, all of the bolts provided will be able to participate virtually uniformly in the transmission of the braking torque occurring at the brake disc, even without the bolts experiencing any plastic or wear-induced deformations. It is also an object of the present invention to provide an additional method for attaching a nonmetallic hub, as well as to provide a method for manufacturing brake units of this kind.
The present invention provides a brake unit (1) having a nonmetallic brake disc (2) of ceramic and a metallic hub (10, 10xe2x80x2), which radially overlap one another with their mutually facing rims and are mechanically joined by a wreath-type rim of mounting bolts (15) that axially penetrate the overlapping rims, in the area of its inner rim, the brake disc (2) having corresponding entrainment bores (5) or radial entrainment slots to receive the mounting bolts (15), wherein the entrainment bores (5) or entrainment slots are lined with a sleeve (20) of plastically deformable material, or the disc material is conceived to be plastically deformable to specific areas in this region. In the brake unit of the present invention having a metal hub, it is the region of the brake disc representing the inside face of the entrainment bore that is designed to be plastically deformable. This can be achieved, on the one hand, with the aid of a plastically deformable sleeve, or by using a plastic form of the normally rigid ceramic material. The effect of the plastic deformability is to compensate for the above-mentioned positional deviation of the bolts, on the one hand, and of the entrainment bore, on the other hand, in that the sleeve undergoes a deformation in response to mechanical stress, so that all bolts are load-bearing and undesired play no longer occurs.
Provision is made for a brake unit of the present invention having a nonmetallic hub for the hub to be molded into an inner region of the ceramic brake disc and doped with boron.
A soft metal sleeve may be used as a plastically deformable lining for the inside bore face. Sleeves of copper or aluminum, for example, are suitable. Instead of a metal sleeve, a sleeve of carbon fiber-reinforced plastic (CFK material) or carbon fiber-reinforced carbon (C/C material) may also be used. A nonmetallic CFK or C/C material of this kind is sufficiently plastically deformable. The sleeve is preferably slotted at one peripheral location over the entire length and fits with mechanical prestressing on the inside face of the bore or of the slot. The longitudinal slot makes the sleeve radially elastic, so that even under conditions of considerable temperature variations, accompanied by noticeable expansion or contraction, it fits with prestressing on the inner bore face.
The wall thickness of the sleeve is a function of the size of the brake disc or of the diameter of the bores. The wall thickness may typically be {fraction (1/20)} of the bore diameter. In the case of motor vehicle brake discs having a bore diameter of, for example, 15 mm, the wall thickness is about 0.75 mm; in the case of railroad brake units having a bore diameter of, for example, 200 mm, the wall thickness is about 10 mm.
It is a feature of a further embodiment of the present invention that the area immediately along the inner face of the entrainment bores or of the entrainment slots contains boron and/or boron-containing compounds and/or boron-releasing compounds in a locally limited fashion, this area being made of a non-ceramic C/C material, while the ceramic brake disc is made of C/Cxe2x80x94SiC material. This embodiment is based on the realization that boron or boron-containing or boron-releasing compounds effectively prevent silicon carbide formation and ceramization during liquid siliconization. If the porous C/C preform, which is intended to undergo the liquid siliconization, is doped in certain regions with boron and/or boron compounds, then no ceramization takes place in this region. There is no xe2x80x9csleevexe2x80x9d as a separate structural element in this embodiment.
The same result is also achieved, however, when the entrainment bores or the entrainment slots are lined with a sleeve of carbon fiber-reinforced carbon, which contains boron and/or boron-containing compounds and/or boron-releasing compounds. At its periphery, the sleeve makes the transition to silicon carbide and thus forms a direct, or material-to-material bond with the C/Cxe2x80x94SiC brake disc.
In both cases, the bore obtained has an inner face of C/C material, which gradually changes over to ceramic. Depending on the size of the brake disc or of the diameter of the entrainment bores or the entrainment slots, the radial layer thickness of this C/C material structure is preferably about {fraction (1/20)} of the bore diameter, in the case of motor vehicle brake discs, for example, from about 0.7 to 2.0 mm, in particular about 1.0 mm. Within these ranges, the boron and/or the boron-containing compounds are present in a concentration of from 5 to 35% by weight. Besides boron itself, suitable compounds are boron carbide, boron nitride, boron tetrachloride and/or borides, in particular zirconium diboride, hafnium diboride, silicon tetraboride and/or titanium diboride, as well as boroncontaining polymers, in particular polyborosilazanes.
In principle, there are two different ways to couple the hub to the brake disc. The entrainment bores or the entrainment slots may be configured in the area of the inner rim of the brake disc, and hub may be provided with an outwardly projecting, flange-type rim that joins it to the inner rim of the brake disc. The coupling may also be achieved by configuring the entrainment bores in the inside area of the brake disc, and by joining the hub by its free end to the inner side of the brake disc.
A brake unit of the present invention that includes a ceramic brake disc and a nonmetallic hub has a brake disc of fiber-reinforced ceramic, preferably a C/Cxe2x80x94SiC brake disc and a hub of the same fiber matrix, preferably a C/C hub. The hub is doped with boron and molded over an outwardly-projecting, flange-type rim into the inner side of the brake disc. Thus, the hub is made of a material which is not as damage-prone as the ceramic material of the brake disc, is less brittle, and easier to machine. The material permits greater expansion. On the other hand, a rigid hub is restricted in its use. Thus, the present invention provides for the hub to be integrally formed in the brake disc. An attachment of this kind provides for a high load-bearing capacity, for example, for braking maneuvers, since there is no longer a xe2x80x9crupture jointxe2x80x9d, as is the case with the prior interconnection layer.
In the method of the present invention for producing molded articles from fiber reinforced C/Cxe2x80x94SiC composite ceramic preforms are fabricated, which are doped with boron and/or with the mentioned boron compounds in those regions that are not designated for ceramization. The preforms can either be green compacts (thus preforms of CFK material) or the C/C bodies that result from pyrolysis of the green compacts.
Alternatively, fabricate green compacts, which correspond to the particular intended partial areas, may be fabricated as separate preforms. The green compacts corresponding to the partial areas not designated for ceramization, or the C/C members resulting from pyrolysis thereof, are doped using boron and/or the mentioned boron compounds and are joined, at the latest prior to the siliconization, to form the desired molded articles.
An advantage of the first alternative is that doping the bore regions eliminates the need for the entrainment bores to be drilled prior to the liquid siliconization. Rather, the brake disc can be mechanically machined in a uniform fashion, with respect to the entrainment bores as well, subsequent to the ceramization.
A second alternative of the method of the present invention provides for the entrainment bores to be drilled, at the latest, before the liquid siliconization takes place. Boron-doped sleeves or solid bodies are then inserted into the bores. This alternative provides easy control of the infiltration depth and boron doping. The initial products are either green compacts (i.e., CFK bodies) or already pyrolyzed, non-ceramic C/C bodies, in any desired combination. Thus, CFK brake discs can be combined with CFK- or C/C-sleeves or solid bodies. C/C brake discs may also be combined with CFK- or C/C-sleeves or solid bodies. The sleeves or solid bodies may be doped both in the CFK phase as well as in the C/C phase.
During the liquid siliconization, only a small portion of the silicon penetrates into the rim area of the now pyrolyzed sleeve or of the solid body and thus leads to a direct bonding between the sleeve or solid body and brake disc. Due to the boron doping, the doped sleeve or the doped solid body is not siliconized, but rather remains as a non-ceramic C/C workpiece. If a solid body is used, it needs only to be subsequently provided with a bore. A solid body or a bolt is more easily fabricated than a sleeve, and is less sensitive to handling. A benefit of the method of the present invention is that partial areas of a molded article can be targeted for ceramization. The boron-doped partial areas remain as non-ceramic areas, and are able to be more easily reworked. They permit a greater expansion than the ceramized partial areas, are not damage-prone, and are less brittle.
A method of the present invention is, therefore, suited for manufacturing one-piece brake units of a brake disc and of a non-metallic hub when the brake disc is a C/Cxe2x80x94SiC brake disc and the hub is a C/C hub. In this case, the brake disc and the hub are manufactured separately, for example, as green compacts, with the green compact to be machined into the hub being doped with boron. The two parts are then joined, pyrolyzed, and ceramized, i.e., siliconized. The hub escapes the ceramization process because of the boron doping and, thus, remains moldable as a C/C component, while the brake disc is ceramized, the silicon penetrating into the rim area of the hub and, in this manner, ensuring a direct bonding. The green compacts are preferably joined by premolding the brake disc directly on the flange-type rim of the hub.
Another embodiment of a method for manufacturing nonmetallic brake units provides for two preforms for the brake disc. This is particularly advantageous when working with ventilated brake discs which have two disc halves interconnected by crosspiece segments. The two disc halves and the hub are fabricated separately as green compacts and pyrolyzed into C/C molded articles. The two brake disc halves are then joined along a flange-type rim of the hub, and the resulting brake unit is siliconized. Since the hub is doped with boron, it is not ceramized. A small portion of the silicon penetrates, however, into the rim area of the hub and ensures direct bonding.
The boron doping may be undertaken using a plurality of methods. On the one hand, boron and/or the mentioned boron compounds may be worked into the green compact as fillers. On the other hand, the porous C/C molded articles may be partially saturated with boron and/or boron compounds, or coated along the pore walls. The boron or the boron compounds may be introduced out of the liquid phase or out of the gas phase under high pressure.